Factory five that does cobra kit cars has come up with a kit supercar that costs only 20K from them, has a curb weight of like 2250, mid-engine layout, and only requires a c5 vette donor car and g50 transaxle. They say you can be said and done for about 40k, but i've seen wrecked c5's for 4-8K and add a g50 you could have a supercar for 30-35K. Oh and not to mention you start with 350hp on 2250 pounds and the good ole' ls1 is good for around 450hp and more torque and throttle response than you'd ever need in a light car for 1500 bucks.
http://www.factoryfive.com/table/ffrkits/GTM/GTMkit.html
http://www.factoryfive.com/table/ffrkits/GTM/GTMkit.html